This invention relates to high temperature ceramic superconductors. More particularly, it relates to multifilamentary superconductor structures that include a multiplicity of thin and uniform filaments; and to the manufacture of such structures from near net shape precursors.
Superconductors are materials having essentially zero resistance to the flow of electrical current at temperatures below a critical temperature, Tc. A variety of copper oxide ceramic materials have been observed to exhibit superconductivity at relatively high temperatures, i.e., above 77K. Since the discovery of the first copper oxide based superconductor about ten years ago, these superconducting ceramics have attracted wide interest, and their physical and chemical properties have been widely studied and described in many publications.
Composites of superconducting materials and metals are often used to obtain better mechanical and electrical properties than superconducting materials alone provide. These composites are typically prepared in elongated wires, elements and cables by a variety of known processes such as the well-known powder-in-tube (xe2x80x9cPITxe2x80x9d) process in which a metal container is filled with a precursor powder and the filled container is then deformed and thermomechanically processed to form filamentary composites having the desired superconducting properties, and a variety of coated conductor (xe2x80x9cCCxe2x80x9d) processes in which a superconductor material or a precursor thereof is deposited on a substrate which is then further processed to form a composite including a superconducting filament. However formed, a multiplicity of filaments may be bundled and/or cabled, with additional deformation and thermomechanical processing steps as needed, to provide multifilamentary composites.
To be commercially viable, high-temperature superconductor (HTS) wire must have high performance (e.g., high critical current density of the superconductor, Jc) and low cost. In the past, considerable efforts have been directed to improving the Jc of superconducting ceramics through densification and crystallographic alignment or texture of the superconducting grains; more recently, there has been increasing interest in and efforts to develop manufacturing technologies through which long lengths of HTS wires can be fabricated with higher, and commercially acceptable, price to performance (as measured by $/kA m) ratios.
At this time, it is known in the HTS community that the highest performing BSCCO (both 2212 and 2223) contain highly aspected (an xe2x80x9caspectedxe2x80x9d element has, in transverse cross-section, a width greater than its height) filaments with dimensions on the order of 10 xc3x97100 microns, and that composite Bi-2223 conductors fabricated using PIT techniques can achieve relatively high Jc performance if asymmetric deformation resulting in an aspected element is employed. For example, using asymmetric deformation, a Jc value of 69,000 A/cm2 at 77K and self field has been reported (Q. Li et al., Physica C, 217 (1993) 360); and it has been predicted that the Jc performance of Bi-2223 conductors may be improved drastically if the thickness of the superconducting layer is decreased from the 30 micron level used by Li et al. to the three micron level. A Jc value in excess of 100,000 A/cm2 (77K, 0 T) has been estimated for the Bi-2223 layer (about 1.5 micron thick) that is immediately adjacent to the Ag in conventionally fabricated elements. Other HTS wire types have shown short length performance, e.g., coated conductors based on Y-123 which are fabricated using thin film techniques using such equipment as vacuum systems, lasers and ion guns.
It is difficult to achieve filament thicknesses in the range of 3 microns using conventional PIT techniques in which axisymmetric deformation is used to prepare a round multifilamentary precursor that is subsequently rolled into a highly aspected element, for two principal reasons. First, the strain path for each filament is a function of its position within the composite, and filaments in the edges of the final element will be less textured and will have a lower performance level than those in the central region of the element. Second, the pre-deformation cross-section of each filament is typically circular, and it is difficult to achieve a thin and wide filament by deforming an initially round filament.
A variety of deformation processing procedures have been proposed. Copending application Ser. No. 08/468,089, filed Jun. 6, 1995 now U.S. Pat. No. 6,247,224 entitled xe2x80x9cSimplified Deformation-Sintering Process for Oxide Superconducting Articlesxe2x80x9d, and incorporated herein by reference in its entirety, describes a method for preparing a highly textured oxide superconductor article in a single, rather than a multiple step, deformation-sinter process. In the procedure described a precursor article, including a plurality of filaments extending along the length of the article and comprising a precursor oxide having a dominant amount of a tetragonal BSCCO 2212 phase and a constraining member substantially surrounding each of the filaments, is subjected to a heat treatment at an oxygen partial pressure and temperature selected to convert a tetragonal BSCCO 2212 oxide into an orthorhombic BSCCO 2212 oxide. Thereafter, the article is roll worked in a single high reduction draft in a range of about 40% to 95% in thickness so that the filaments have a constraining dimension is substantially equivalent to a longest dimension of the oxide superconductor grains, and is then sintered to obtain a BSCCO 2212 or 2203 oxide superconductor. Other procedures are disclosed in copending application Ser. No. 08/651,688, filed Nov. 11, 1995 and entitled xe2x80x9cImproved Breakdown Process for Superconducting Ceramic Composite Conductorsxe2x80x9d, which application is also here incorporated by reference in its entirety.
To be practical outside the laboratory, most electrical and magnetic applications require flexible cabled lengths of conductor manufacturable with high fill factors (i.e. a high volume percent of superconductor in the composite multifilament structure) in addition to high current-carrying capacity. Thus, in addition to making individual filaments with high Jc, considerable effort also has been directed to the manufacture of cables and the like which include a multiplicity of HTS filaments. For example, copending application Ser. No. 08/554,814, filed Nov. 11, 1995, now U.S. Pat. No. 6,247,225, entitled xe2x80x9cCabled Conductors Containing Anisotropic Superconducting Compounds and Method for Making Them,xe2x80x9d and also hereby incorporated by reference in its entirety, discloses a cabled conductor comprising a plurality of transposed strands each comprising one or more preferably twisted filaments preferably surrounded or supported by a matrix material and comprising textured anisotropic superconducting compounds which have crystallographic grain alignment that is substantially unidirectional and independent of the rotational orientation of the strands and filaments in the cabled conductor. The cabled conductor is made by forming a plurality of suitable composite strands, forming a cabled intermediate from the strands by transposing them about the longitudinal axis of the conductor at a preselected strand lay pitch, and, texturing the strands in one or more steps including at least one step involving application of a texturing process with a primary component directed orthogonal to the widest longitudinal cross-section of the cabled intermediate, at least one such orthogonal texturing step occurring subsequent to said strand transposition step. In one embodiment, the filament cross-section, filament twist pitch, and strand lay pitch are cooperatively selected to provide a filament transposition area which is always at least ten times the preferred direction area of a typical grain of the desired anisotropic superconducting compound. For materials requiring biaxial texture, the texturing step may include application of a texturing process with a second primary component in a predetermined direction in the plane of the widest longitudinal cross-section of the conductor.
Others, e.g., U.S. Pat. 5,508,254, have proposed forming a multifilamentary structure by vertically stacking relatively thick rolled tapes.
However, and despite all of the past and ongoing work in the field, both cost and performance are still major constraints limiting the widescale use of HTS wires in the marketplace. There remain the needs to increase the Jc of HTS filaments, to provide multi-filament composites of varying geometry having greater fill factors and overall current-carrying capacity, and to accomplish all of this at reduced cost.
The invention features a multi-filamentary superconductor having a high fill factor (e.g., greater than 30% and preferably greater than 35% to 40%) which is made in a semi-continuous procedure from a number of superconductor precursor elements, each of which has substantially the same overall geometry and which include superconductor precursor monofilaments having the same overall configuration. The superconductor precursor monofilaments are provided on or in a metal component. Before rolling, the precursor monofilaments have a low density (i.e., in the range of 25 to 70 percent, preferably 30-65 percent, and most preferably 40 to 60 percent, theoretical density); after rolling, the thickness of the monofilaments is not more than about 50 microns (and preferably not more than about 40 microns). The elements are consolidated into a composite in which the spatial relationship of the elements is such that all of the elements are symmetric relative to each other and also both to the external shape of the composite and to subsequent deformation. In the consolidated precursor composite, metal components of the composite form a bonded ladder structure with superconductor precursor monofilaments in the space between adjacent xe2x80x9crungsxe2x80x9d. Both before and after consolidation the configurations of the filaments of the different precursor elements are substantially the same.
As used herein, xe2x80x9cprecursorxe2x80x9d means any material that can be converted to a desired anisotropic superconductor upon application of a suitable heat treatment. If the desired anisotropic superconductor is an oxide superconductor, for example, precursors may include any combinations of elements, metal salts, oxides, suboxides, oxide superconductors which are intermediate to the desired oxide superconductor, or other compounds which, when reacted in the presence of oxygen in the stability field of a desired oxide superconductor, produces that superconductor. Whatever the particular precursor, in the practice of the present invention, the final aspect ratio of the composite, and of the superconductor monofilaments in it, may be decoupled from the aspect ratios of the individual precursor elements and superconductor precursor filaments.
xe2x80x9cConsolidatexe2x80x9d, as used herein, means to carry out operations that allow an assembly of elements to behave as a unit, at least to the extent that there is no large scale displacement of various elements of the assembly (e.g., the precursor composite) during subsequent processing. Consolidation may be accomplished by a number of procedures including heat treatment (thermal processing), chemical adhesion, and drawing or other deformation processes. The preferred procedure includes sufficient thermal heating to accomplish an initial phase transition of the superconductor precursor.
In one aspect of this invention, the composite precursor includes a number of precursor elements, each of which includes at least one HTS precursor monofilament, stacked in side by side alignment to form a layer in which the tops and bottoms of the elements and also the filaments in the elements are generally aligned across the width of the composite. Metal (typically a noble metal although other metals, with a buffer layer as required to prevent interreaction with the superconductor components, may also be used) is provided between each adjacent pair of precursor filaments. In embodiments of this aspect, the composite may include more than one layer of side-by-side precursor elements, in which event the layers are stacked vertically in such a way that each HTS precursor filament is either in vertical alignment with or is centered on the thin space between HTS precursor filaments in any other layer. All the HTS precursor filaments in the different precursor elements have essentially the same aspect ratio, width and thickness. The stacked composite structure typically is provided with a surrounding metal wrap or sheath and heated to consolidate its various components.
In a second aspect of the present invention, each precursor element includes metal having a layer of an HTS precursor deposited on at least one face of the metal, and preferably on both of its opposite faces, and in the composite metal overlies both an otherwise exposed face and the side edges of each of the HTS precursor layers.
In preferred practices of the invention, precursor composites are thermomechanically processed in a 1DS or 2DS procedure (as discussed hereinafter) in which the element is reduced by about 40% to about 95% in thickness (with no subsequent reduction in thickness in excess of about 5% prior to a sintering step), and the thus-rolled precursor composite is sintered to obtain a final superconductor composite structure having effectively uniform filaments. In high performance structures, the HTS filaments are typically less than about 10 microns, preferably in the range of about 2-7 microns, and most preferably about 5 microns, thick.
Preferred precursor composites are made in PIT or coated conductor processes in which a plurality of effectively identical superconductor precursor elements having low density HTS precursor monofilaments are (after drawing but before rolling for PIT elements) stacked and surrounded with a supporting fine-grain metal. In each precursor element, the HTS precursor filament is on or in a fine grain metal. In PIT processes, the element is drawn to smaller size in a procedure involving frequent anneals to maintain the fine grain size and deformation characteristics of the metal. By xe2x80x9cfine-grainedxe2x80x9d is meant an average grain size that is typically less than 300 micrometers, preferably less than 200 micrometers, more preferably less than 100 micrometers, even more preferably less than 50 micrometers, or, most preferably less than 20 micrometers. The maximum grain size is typically less than about 300 micrometers, preferably less than about 200 micrometers, more preferably less than about 100 micrometers, and most preferably less than about 50 micrometers. The stacked precursor elements and support are then consolidated and thermomechanically processed. The precursor elements may be made using a PIT process in which each element is made from a metal tube filled with precursor powder to a low density which then has been drawn to provide a structure not more than about 600 microns in diameter and in which the superconductor precursor monofilament is of low density and not over about 300 microns thick. Such drawn elements may then be rolled into high aspect ratio elements, which depending on the procedure used may result in high density HTS precursor filaments. The precursor elements also may be made using a deposition/coating process in which each element comprises a fine-grain metal substrate carrying a low density precursor superconductor layer. In the latter event, a precursor superconductor layer is preferably provided on both sides of the substrate, and the composite precursor includes fine grain metal between adjacent precursor elements.